This invention relates to a method of preparing a optical fiber preform with the preform having a uniform refractive index profile for the deposition material that ultimately forms the core of the optical fiber. More particularly, the invention relates to a method of removing or reducing non-uniform regions in the deposition material on the surface of the preform formed during the processing of the preform or by exposure to certain environmental conditions, which result in the preform having a uniform refractive index profile for the deposition material that ultimately forms the core of the optical fiber.
Optical fibers have acquired an increasingly important role in the field of communications, frequently replacing existing copper wires. This form of transmission is done by sending a beam of light through an optical fiber. Interference with the light beam or its partial loss during transmission must be at a minimum to make the use of optical fibers a successful communications technology. The manufacture of optical fibers used for communications is a complicated and time intensive process involving many steps. Each step is another point in the manufacturing process in which defects can be introduced into the product.
Typically an optical fiber consists of a core and cladding. The core is used to propagate the light rays, and the cladding is used to contain (through reflection) the rays within the core. Defects in the core (and materials used to form the core) are critical since these defects can hinder the propagation of the light rays resulting in loss or attenuation of the light through the fiber and therefore a decrease in the distance light can be propagated without being amplified. There are a number of steps in the manufacturing of an optical fiber which are used to reduce or minimize defects in the fiber, and in particular in the deposition material to be used for the core of the fiber.
Optical fibers can be formed from preforms by drawing a fiber therefrom. Preforms can take the form of a hollow, tube shaped glass. Such preforms can be made by a process in which a vitreous material is deposited on an internal and/or external surface of a glass tube. The number of layers of the deposited material, the composition of the deposited material and the surface(s) of the glass tube on which the material is deposited are determined based on the type of fiber to be manufactured include but are not limited to step-indexed multimode, graded-index multimode, step-index single-mode, dispersion-shifted single-mode, and dispersion-flattened single-mode. Examples of processes suitable for forming a preform include the outside vapor deposition (OVD), vapor axial deposition (VAD), and inside vapor deposition processes such as modified chemical vapor deposition (MCVD) and plasma assisted chemical vapor deposition (PCVD). Once a material is deposited on a glass tube with the desired profile, an optical fiber can be drawn from the preform after the preform is collapsed into a solid rod. Collapsing the preform presents an important advantage over other processes since solid preforms can be stored indefinitely without contaminating the inner layers, which will become the light propagating cores of the optical fibers. Collapsing the preform, however, also presents a drawback in that during the collapsing step volatile dopants in the deposited material such as germanium are desorbed or released from the material. These dopant molecules are then either re-deposited at another location or transported out of the preform. The transport and/or re-deposition of the dopants out of the material deposited on the preform results in a refractive index deviation at the center of the core of the optical fibers pulled from such a preform. The refractive index deviation takes the form of spikes or dips.
One of the steps in manufacturing an optical fiber is to remove defects such as impurities or depleted regions at or near the surface of the deposited material including those caused by environmental contamination such as by for example re-wetting or by depletion of the dopant from the surface of the deposited material. This step which is commonly employed in inside vapor deposition processes such as MCVD and PCVD involves etching the inner surface of the preform. The etching can take place on a un-collapsed preform or on a partially collapsed preform. During the etching step, etchant gases containing fluorine are used to remove deposited material from the preform surface. It has been found, however that if the etching takes place at too high a temperature or where the concentration of the gas containing fluorine is too high, then even if the original defects are removed through etching, further defects from fluorine contamination can be incorporated into the surface of the deposited material. This may result in a decrease in the refractive index in the core of the fiber, but certainly not in an elimination of that defect. To solve this problem, it was found by others that certain combinations of lowering the etching temperature, using less aggressive etchant gases and lowering the concentration of those less aggressive etchant gases can reduced or eliminated fluorine contamination during etching. This, however, results in a slower, time consuming processing step which is also less desirable.
This present invention is directed to a method of preparing an optical fiber preform such that the preform has a more uniform refractive index profile across the deposited glass material that ultimately forms the core of the optical fiber. This method preferably results in the removal or reduction of non-uniformly doped regions in the deposited material on the surface or centerline of the preform. Such non-uniformly doped regions can be formed during the processing of the preform, or by exposure to certain environmental conditions, which result in the preform having a uniform refractive index profile across the deposited material that ultimately forms the core of the optical fiber.
In one embodiment, an optical fiber preform is etched a first time to remove a portion of a deposited oxide material from the preform by using a gas comprising an etchant gas containing fluorine at a sufficient temperature and gas concentration to create a fluorine contamination layer in the remaining oxide material. The preform is then etched a second time using a gas comprising an etchant gas at a sufficient temperature and gas concentration to remove the fluorine contamination layer which is present in the remaining oxide material, without any substantial further fluorine contamination of the remaining oxide material. Preferably the second etchant gas also contains fluorine.
In another embodiment, an optical fiber preform is prepared which comprises an inner and outer surface and comprises at least one oxide material deposited on the inner surface of the preform. The deposited oxide material comprises a dopant. The glass preform is collapsed under conditions which result in the depletion of some of the dopant out of the deposited oxide material for a distance from the inside surface. The glass preform is subsequently etched a first time to remove part or all of a dopant depleted layer from the deposited oxide material using a gas comprising an etchant gas containing fluorine at a sufficient temperature and gas concentration to create a fluorine contamination layer in the remaining deposited oxide material. The glass preform is etched a second time using a gas comprising an etchant gas containing fluorine at a sufficient temperature and gas concentration to remove any remaining dopant depleted layer and the fluorine contamination layer without any substantial further fluorine contamination of the remaining deposited oxide material.
In still another embodiment, an optical fiber preform is formed via a process which comprises the steps of a) partially collapsing a glass tube with an inner and outer surface and comprising a deposited oxide material on the inside of the tube, the deposited oxide material comprising a dopant, under conditions which results in the depletion of some or all of the dopant from a region of the deposited oxide material for a distance from the inner surface; b) flowing a mixture of gas comprising an etchant gas containing fluorine over the inner surface of the glass tube at an etchant gas flow rate of greater than about 65 sccm and a temperature of less than 1700xc2x0 C.; and c) flowing a second mixture of gas comprising an etchant gas containing fluorine at a sufficient temperature and gas concentration to remove any remaining dopant depleted layer and fluorine contamination resulting from step b) without any substantial further fluorine contamination of the remaining deposited oxide material.
Additional features and advantages of the invention will be set forth in the detailed description which follows, and in part will be readily apparent to those skilled in the art from that description or recognized by practicing the invention as described herein, including the detailed description which follows, the claims, as well as the appended drawings.
It is to be understood that both the foregoing general description and the following detailed description are merely exemplary of the invention, and are intended to provide an overview or framework for understanding the nature and character of the invention as it is claimed. The accompanying drawings are included to provide a further understanding of the invention, and are incorporated in and constitute a part of this specification. The drawings help to better illustrate the invention, and together with the description serve to explain the principles and operation of the invention.